The present invention relates to injection molding, and in particular to two-step injection molding for forming plastic articles having wall portions relatively thinner than would be achieved with standard injection molding techniques.
Plastic articles formed by injection molding have a wide variety of uses. However, it is essential in injection molding that the injected molten plastic flow evenly throughout the entire mold to properly form the article. It has been found that such molten plastic will ordinarily not flow through a section of the mold which is less than 0.015 inch which limits the thinness of the walls of an article formed by injection molding. This limitation is often critical when it is desired to form plastic articles which have extremely thin walls or wall portions.
One example of an application wherein it is desired to form a plastic article having an extremely thin wall portion is in the plastic cups used with ultrasonic nebulizers. Ultrasonic nebulizers have been developed wherein a solution is poured into a plastic cup and ultrasonic waves are projected upwardly through the base of the cup to nebulize the solution. Such nebulizers are often used to give medicines to persons enclosed in an oxygen tent, eliminating the necessity for injecting the medicine, and are thus quite useful. However, a significant problem wih such nebulizers is the power loss of the ultrasonic waves as they are transmitted through the base of the cup. This power loss is directly related to the thickness of the base of the cup, and attempts have been made to make this base as thin as possible. Currently available injection molding techniques are not capable of producing plastic cups having sufficiently thin bases and the power loss of the ultrasonic waves is thus excessive.
With currently available injection molding techniques, a thin wall portion of an article is constructed simply by making the corresponding portion of the injection mold to the desired reduced thickness. As stated above, it has been found that this thickness should be no less than 0.015 inch or else the plastic will not readily flow through the thin section, and a gap may be left in the plastic article. Attempts have been made to force the plastic through thin wall sections having thicknesses no greater than 0.008 to 0.009 by using specialized molding techniques, and plastic cups have been made for ultrasonic nebulizers using these techniques. However, the quality of these nebulizer cups has been quite poor. The plastic must be heated to a temperature much greater than that normally used for injection molding, causing possible molecular breakdown of the plastic material and nonuniform cooling. Also, moisture in the air tends to be trapped at the thin wall portion of the mold, and condenses and is trapped in the plastic. This problem has been partially solved by adding a dehumidifier to the injection mold, but this solution to the problem is quite complex and is not fully successful. The poor quality of the cups manufactured according to these techniques results in many rejects, and the process is quite slow, greatly increasing the cost of the product. In addition, the plastic is still not as thin as desired, and large power losses still occur through the base of the cup.